An accelerometer is a sensor typically utilized for measuring acceleration forces. These forces may be static, like the constant force of gravity, or they can be dynamic, caused by moving or vibrating the accelerometer. An accelerometer may sense acceleration or other phenomena along one, two, or three axes or directions. From this information, the movement or orientation of the device in which the accelerometer is installed can be ascertained. Accelerometers are used in inertial guidance systems, in airbag deployment systems in vehicles, in protection systems for a variety of devices, and many other scientific and engineering systems.
Capacitive-sensing MEMS accelerometer designs are highly desirable for operation in high gravity environments and in miniaturized devices, due to their relatively low cost. Capacitive accelerometers sense a change in electrical capacitance, with respect to acceleration, to vary the output of an energized circuit. One common form of accelerometer is a capacitive transducer having a “teeter-totter” or “see saw” configuration. This commonly utilized transducer type uses a movable element or plate that rotates under z-axis acceleration above a substrate. The accelerometer structure can measure at least two distinct capacitances to determine differential or relative capacitance.
Referring to FIGS. 1 and 2, FIG. 1 shows a top view of a prior art capacitive-sensing MEMS sensor 20 constructed as a conventional hinged or “teeter-totter” type accelerometer, and FIG. 2 shows a side view of MEMS sensor 20. MEMS sensor 20 includes a static substrate 22 and a movable element 24 spaced from substrate 22, each of which have opposed planar faces. Substrate 22 has a number of conductive electrode elements 26 of a predetermined configuration deposited on a substrate surface 28 to form capacitor electrodes or “plates.” In an exemplary scenario, electrode elements 26 may operate as excitation or sensing electrodes to receive stimulating signals. Electrode elements 26 may additionally operate as a feedback electrodes when a feedback signal is superimposed on the sensing signal.
Movable element 24, commonly referred to as a “proof mass,” is flexibly suspended above substrate 22 by one or more suspension anchors, or rotational flexures 30, for enabling movable element 24 to pivot or rotate about a rotational axis 32 to form capacitors 34 and 36, labeled C1 and C2, with electrode elements 26. Movable element 24 moves in response to acceleration, thus changing its position relative to the static sensing electrode elements 26. This change in position results in a set of capacitors whose difference, i.e., a differential capacitance, is indicative of acceleration in a direction 37.
When intended for operation as a teeter-totter type accelerometer, a section 38 of movable element 24 on one side of rotational axis 32 is formed with relatively greater mass than a section 40 of movable element 24 on the other side of rotational axis 32. The greater mass of section 38 is typically created by offsetting rotational axis 32. That is, a length 42 between rotational axis 32 and an end 44 of section 38 is greater than a length 46 between rotational axis 32 and an end 48 of section 40. In addition, electrode elements 26 are sized and spaced symmetrically with respect to rotational axis 32 and a longitudinal axis 50 of movable element 24.
Many MEMS sensor applications require smaller size and low cost packaging to meet aggressive cost targets. In addition, MEMS sensor applications are calling for lower temperature coefficient of offset (TCO) specifications. The term “offset” refers to the output deviation from its nominal value at the non-excited state of the MEMS sensor. Thus, TCO is a measure of how much thermal stresses effect the performance of a semiconductor device, such as a MEMS sensor. A high TCO indicates correspondingly high thermally induced stress, or a MEMS device that is very sensitive to such stress. The packaging of MEMS sensor applications often uses materials with dissimilar coefficients of thermal expansion. Thus, an undesirably high TCO often develops during manufacture or operation. These thermal stresses, as well as stresses due to moisture and assembly processes, can result in deformation of the underlying substrate 22, referred to herein as package stress.
Referring to FIGS. 3-4, FIG. 3 shows a cross-sectional edge view of MEMS sensor 20 along section lines 3-3 in FIG. 1, and FIG. 4 shows a cross-sectional edge view of MEMS sensor 20 along section lines 4-4 in FIG. 1. A problem particular to the teeter-totter configuration shown in FIG. 1 is that when teeter totter configuration of MEMS sensor 20 is subject to a bending moment from substrate 22 caused by package stress, the stress causes section 40, i.e., the lighter section, to deform more than section 38, i.e., the heavier section, resulting in an offset change. As illustrated in FIGS. 3 and 4, package stress can result in deformation of section 40 of movable element 24 that is significantly greater than the deformation of section 38 of movable element 24. This non-symmetric bending induced by package stress can result in an undesirably high offset difference between sense capacitances 34 and 36 (i.e., poor TCO performance), thus adversely affecting capacitive accelerometer 20 output.
Thus, what is needed is a low cost, compact, single die teeter-totter type MEMS sensor that can sense along one or more axes and is less susceptible to thermally induced package stress gradients.